1. Field of the Invention
The present invention relates to an extrusion die for honeycomb extrusion molding and a manufacturing method therefor.
2. Description of the Related Art
A conventional automobile exhaust gas purifying catalyst is what is called a honeycomb catalyst in which a catalyst component is carried on each cell surface of a ceramic honeycomb carrier (honeycomb structure). Since the strength in the axial direction thereof is higher than the strength in the cross-sectional (radial) direction, a construction in which the honeycomb carrier is held in the axial direction has been used. In this case, to prevent breakage occurring near the outer peripheral portion when the honeycomb carrier is held in the axial direction, cell walls (ribs) in the outer peripheral section is made thicker than those in the inside section, by which the axial compressive strength of the honeycomb carrier is increased.
However, in recent years, a decrease in pressure loss in the honeycomb catalyst has been required in response to a tendency for higher engine output, and the effective use of the whole of catalyst carrier has been required in response to strengthened exhaust control. To meet these requirements, a construction has begun to be used in which the honeycomb catalyst carrier is not held in the axial direction, but is mainly held on the outer peripheral surface of the honeycomb catalyst carrier. One reason for this is that since the volume of catalyst is increased and the mass of catalyst is increased by the strengthened exhaust control, the axial holding cannot hold the honeycomb carrier sufficiently against engine vibrations because of a small holding area.
Also, on the other hand, to increase the purifying performance of catalyst, a move for decreasing the heat capacity of catalyst and improving the warm-up characteristics of purifying performance has been started by decreasing the cell wall thickness of honeycomb carrier to reduce the weight of honeycomb carrier.
Therefore, there is a tendency for the fracture strength against the external pressure from the outer peripheral surface of honeycomb carrier to be further decreased by the thinner cell wall.
Furthermore, since the exhaust control has recently been strengthened further, the temperature of exhaust gas has increased year by year to improve the engine combustion conditions and to increase the catalyst purifying performance. Accordingly, the requirement for thermal shock resistance of the honeycomb carrier has been made stringent.
Thus, the thinner cell wall, the use of holding method of outer peripheral surface of honeycomb carrier, and the increase in exhaust gas temperature have presented big problems of the setting of thickness of cell wall and honeycomb external wall, the increase in isostatic strength of honeycomb structure, and the high accuracy of outside shape and wall shape.
In view of the above situation, Japanese Patent Application No. 2000-236122 has proposed a ceramic honeycomb structure 1 shown in FIGS. 9 and 10.
As shown in FIG. 9, the ceramic honeycomb structure 1 is made up of a plurality of adjoining cell walls (ribs) 2 forming a cell composite and an external wall 4 which surrounds and holds an outermost cells located at the outermost periphery of the cell composite, and is composed of a composite of a plurality of through holes (cells) partitioned by the cell walls 2.
Also, as shown in FIG. 10, the ceramic honeycomb structure 1 has outermost peripheral cells 8 located closest to the external wall 4, and second cells 9 inwardly from the outermost peripheral cells 8 are continuous. The cell walls 2 are broadly divided into outer peripheral cell walls 2a having a large wall thickness and basic cell walls 2b having a small wall thickness.
Thereby, in comparison with the conventional ceramic honeycomb structure, the above-described ceramic honeycomb structure can realize well-balanced harmony between disadvantages of increased pressure loss and decreased thermal shock resistance and advantages of increased isostatic strength and highly accurate wall and honeycomb structure shapes, so that this ceramic honeycomb structure is anticipated as an automobile exhaust gas purifying catalyst carrier or the like.
An extrusion die used when the above-described honeycomb structure is extrusion molded is, for example, one as shown in FIG. 1. Usually, a slit narrow portion (for example, 2 mil [about 0.05 mm]) of an inside section 22 is machined by grinding or wire electrical discharge, and a slit wide portion (for example, 3 mil [about 0.075 mm]) of an outer peripheral section 24 is machined by electrical discharge using a carbon electrode.
However, for the above-described extrusion die 10, since the slit width of the inside portion 22 and the slit width of the outer peripheral section 24 are different from each other, if the slits are machined only by grinding using a disk-shaped grinding stone, in the vicinity of the boundary between the inside section 22 and the outer peripheral section 24, a locus (hatched portion in FIG. 5(b)) due to grinding stone cut depth and contact arc is drawn, for example, as shown in FIG. 5(b), and a difference is made between the slit depth L1 of the wide portion in the X direction and the slit depth L2 of the wide portion in the Y direction.
Therefore, the ceramic honeycomb structure which is extrusion molded by using the above-described extrusion die has a problem in that cell deformation defects 30 are produced at the boundary between the outer peripheral cell wall 2a and the basic cell wall 2b as shown in FIGS. 6 and 7.
Also, the slit wide portion of the outer peripheral section 24 is machined by electrical discharge using a carbon electrode, and particularly when an extrusion die with a slit width of 2 to 3 mil (about 0.05 to 0.075 mm) or narrower is manufactured, the machining accuracy (±2 to 3 μm) of slit width is insufficient. Also, there is a difference in surface roughness of machined surface between grinding and electrical discharge machining. Therefore, the ratio of surface roughness of the outer peripheral section 24 to the inside section 22 is as high as 10 and more, which presents a problem of poor extrusion pattern of extrusion die.
Further, for the above-describe extrusion die 10, since the slit width in the inside section 22 is different from the slit width in the outer peripheral section 24, if extrusion molding is performed as it is, the molding speed in the outer peripheral section 24 is higher than the molding speed in the inside section 22 depending on the flowability of a raw material to be molded. Therefore, there arises a problem in that the pattern is curled, and thus a defective honeycomb structure 50 is liable to be produced as shown in FIG. 8.